1. Field of the Invention
The present invention relates to a device for detecting knocking in an internal combustion engine by using ionic current. More particularly, the invention relates to a device, for detecting knocking in an internal combustion engine, which does not erroneously detect knocking even when spike noise is generated.
2. Prior Art
In an internal combustion engine using gasoline as a fuel, a gas mixture compressed by a piston is ignited by a spark plug and is burned to produce an output. That is, in normal combustion, a flame nucleus in a gas mixture is formed near the gap of the spark plug, and propagates over the whole combustion chamber.
The ignition timing of the spark plug has an intimate relationship with the output of the internal combustion engine. When the ignition timing is too late, the propagating speed of flame becomes slow. Therefore, the combustion becomes slow resulting in a decrease in the combustion efficiency and, hence, in a decrease in the output of the internal combustion engine.
When the ignition timing is too early, on the other hand, the propagation of frame is fast, whereby a maximum pressure of combustion rises and the output of the internal combustion engine increases. When the ignition timing is too early, however, there takes place knocking in which the mixture is self-ignited prior to the propagation of the flame, often damaging the internal combustion engine.
That is, it is advantageous to operate the internal combustion engine in a region where the ignition timing is set just before the occurrence of knocking (MBT: minimum spark advance for best torque) from the standpoint of fuel efficiency and output. It is very important to reliably detect the occurrence of knocking.
A knock sensor which is a vibration sensor has heretofore been used for detecting knocking. However, a device has been studied which detects knocking by utilizing the phenomenon that ions are generated in the combustion chamber due to the combustion of the mixture and an ionic current flows.
FIG. 1 is a diagram schematically illustrating an ignition circuit for the internal combustion engine, wherein an end of a primary coil 111 of an ignition coil 11 is connected to the positive electrode of a battery 12. The other end is grounded via the collector and the emitter of a switching transistor 13 included in an igniter.
The base of the transistor 13 is connected to an ignition timing control unit 14, so that the transistor 13 is turned on when an ignition signal IGT is output from the ignition timing control unit 14.
An end of a secondary coil 112 of the ignition coil 11 is also connected to the positive electrode of the battery 12, and the other end is connected to a spark plug 8 through a reverse-current preventing diode 15, a distributor (not shown) and a high-tension cable 18.
An ionic current detecting unit 17 is connected to the other end of the secondary coil 112 of the ignition coil 11 in parallel with the spark plug 16.
The ionic current is supplied, through a protection diode 171, to a series circuit of a current-to-voltage converting resistor 172 and a bias power source 173. A voltage generated at a point where the current-to-voltage conversion resistor 172 and the protection diode 171 are connected together, is applied to an amplifying circuit 175 comprised of an operational amplifier and a resistor through a capacitor 174 for removing a DC component.
Therefore, a voltage signal proportional to the AC component of the ionic current is output at an output terminal 176 of the ionic current detecting unit 17.
FIGS. 2A to 2E are diagrams of waveforms at each of the portions of the ignition circuit (FIG. 1) and show, respectively, an ignition signal IGT, a voltage on the grounding side of the primary coil (point P), a voltage on the high-tension side of the secondary coil (point S), and a voltage input to the ionic current detecting unit (point I). All abscissa represent time.
When the ignition signal IGT turns to the "H" level at t.sub.1, the transistor 13 is turned on and the voltage at point P drops. Immediately after t.sub.1, a negative high-voltage pulse is generated at point S, that is, on the high-voltage side of the secondary coil. However, the current is blocked by the reverse current-preventing diode 15 from flowing into the spark plug 16 and the ionic current detecting unit 17.
When the ignition signal IGT turns to the "L" level at t.sub.2 and the transistor 13 is cut off, a voltage at point P abruptly rises, and a positive high-voltage pulse is generated at point S.
The positive high-voltage pulse is not blocked by the reverse current-preventing diode 15 and flows into the spark plug 16 to be discharged. It is prevented by the protection diode 171 from flowing into the ionic current detecting unit 17.
Furthermore, from t.sub.3 to t.sub.4 after the discharge of the spark plug 16, LC resonance is triggered by energy remaining in the ignition coil 11 due to parastic inductance and parastic capacitance of the high-tension cable 18 and the like.
The gas mixture in the cylinder is ignited by the discharge of the spark plug 16, ions are generated in the cylinder as the flame spreads, and an ionic current starts flowing. The ionic current increases with an increase in the pressure in the cylinder and decreases with a decrease in the pressure in the cylinder.
When knocking occurs in the internal combustion engine, knocking signals in a particular frequency band (6 to 7 KHz) are superposed while the ionic current decreases after having reached its peak.
In order to detect the knocking using the ionic current, therefore, it is desired to detect only the knocking signals in particular frequency band and reject other signals (e.g., LC resonance wave). For this purpose, therefore, it is desired to provide a knocking window which opens at t.sub.5 after no extra signal exists and closes at a suitable moment (e.g., ATDC 60.degree.) after the ionic current has decreased, and to detect the knocking based upon the output of the ionic current detecting unit 17 while the knocking window is opened.
"A method of detecting knocking based on an ionic current" has been already proposed (see Japanese Unexamined Patent Publication (Kokai) No. 6-159129). According to this invention, a knocking signal is separated from the output signal of the ionic current detecting unit 17 using a band-pass filter, the separated knocking signals are integrated, and knocking is detected based on the integrated signal.
FIG. 3 is a block diagram according to the above-mentioned publication, wherein the output of the ionic current detecting unit 175 is supplied to a processing unit 34 through a band-pass filter (BPF) unit 32 and an integrating (or peak-holding) unit 33. The operation of the integrating (or peak-holding) unit 33 is controlled by a window which is opened after a predetermined period determined depending upon the engine speed and the load and is closed at a moment corresponding to about 50.degree. CA.
The noise component is rejected by utilizing the fact that the integrated value of noise estimated to be an instantaneous change of the ion concentration stepwisely increases whereas the integrated value of knocking signals increases continuously.
It has also been widely known to provide an LC resonance masking unit 31 between the ionic current detecting unit 17 and the BPF unit 32 in order to reject the effect of LC resonance after the electric discharge.
The ionic current detecting unit 17, however, detects a very small ionic current and must have a very high input impedance and gain, and inevitably picks up the spike noise due to corona discharge of the spark plug 16. Besides, the spike noise has wide frequency spectra and cannot be rejected by the BPF unit 32, and is generated irregularly. Accordingly, it is difficult to reliably separate the spike noise from the knocking, and the spike noise may often be erroneously detected as the occurrence of knocking.
FIGS. 4A to 4E are diagrams explaining the above-mentioned problem, and show, respectively, an output of the LC resonance masking unit 31, outputs of the BPF unit 32 and knocking window, output of the peak-holding unit and output of the integrating unit. All the abscissa represent time.
That is, before t.sub.10, the output of the ionic current detecting unit 17 is masked by the LC resonance masking unit 31, and only after t.sub.11 is a signal output from the LC resonance masking unit. For instance, the knocking window is opened between 10.degree. ATDC and 60.degree. ATDC, and the peak-holding operation or the integration operation is started.
When knocking occurs after 10.degree. ATDC, the output of the peak-holding unit or of the integrating unit increases. When the knocking level is small, however, the output does not exceed the threshold level L.sub.TH.
When a spike noise is generated at t.sub.11, however, the output of the peak-holding unit or the integrating unit may become larger than the threshold level L.sub.TH being affected by the knocking frequency component in the spike noise. In such a case, the processing unit 34 erroneously detects the spike noise as knocking.
When it is erroneously detected that the knocking is occuring though the knocking is not really occurring, the ignition timing is delayed to suppress the knocking, resulting in deterioration of fuel efficiency and output.
The present invention provides a device for detecting knocking of an internal combustion engine, which does not erroneously detect spike noise as knocking even when it is generated.